Datasheet
LT3511
12
3511fc
Leakage Inductance and Clamp Circuits
Transformer leakage inductance (on either the primary or
secondary) causes a voltage spike to appear at the primary
after the output switch turns off. This spike is increasingly
prominent at higher load currents where more stored en-
ergy must be dissipated. When designing an application,
adequate margin should be kept for the effect of leakage
voltage spikes. In most cases the reflected output voltage
on the primary plus V
IN
should be kept below 100V. This
leaves at least 50V of margin for the leakage spike across
line and load conditions. A larger voltage margin will be
needed for poorly wound transformers or for excessive
leakage inductance. Figure 5 illustrates this point. Minimize
transformer leakage inductance.
A clamp circuit is recommended for most applications.
Two circuits that can protect the internal power switch
include the RCD (resistor-capacitor-diode) clamp and the
DZ (diode-Zener) clamp. The clamp circuits dissipate the
stored energy in the leakage inductance. The DZ clamp
is the recommended clamp for the LT3511. Simplicity of
design, high clamp voltages, and low power levels make the
DZ clamp the preferred solution. Additionally, a DZ clamp
ensures well defined and consistent clamping voltages.
Figure 5 shows the clamp effect on the switch waveform
and Figure 6 shows the connection of the DZ clamp.
<100V
<140V
<150V
V
SW
t
OFF
> 400ns
with Clamp
TIME
t
SP
< 150ns
3511 F05
<100V
<150V
V
SW
V
LEAKAGE
t
OFF
> 400ns
without Clamp
TIME
t
SP
< 150ns
Figure 5. Maximum Voltages for SW Pin Flyback Waveform
3511 F06
L
S
D
Z
Figure 6. DZ Clamp
Proper care must be taken when choosing both the diode
and the Zener diode. Schottky diodes are typically the best
choice, but some PN diodes can be used if they turn on
fast enough to limit the leakage inductance spike. Choose
a diode that has a reverse-voltage rating higher than the
maximum switch voltage. The Zener diode breakdown
voltage should be chosen to balance power loss and switch
voltage protection. The best compromise is to choose the
largest voltage breakdown. Use the following equation to
make the proper choice:
V
ZENER(MAX)
≤ 150V – V
IN(MAX)
For an application with a maximum input voltage of 72V,
choose a 68V V
ZENER
which has V
ZENER(MAX)
at 72V, which
will be below the 78V maximum.
The power loss in the clamp will determine the power rat-
ing of the Zener diode. Power loss in the clamp is highest
APPLICATIONS INFORMATION